FFI Support for C Block Objects

Apple recently ​proposed the inclusion of lambda abstractions (closures) into C/C++/Objective-C and facilitated an implementation in the clang compiler framework. They called this language extension blocks (or block objects). It is widely used in the APIs of OS X 10.6 (Snow Leopard) and 10.7 (Lion). This page is about extending the Haskell 2010 FFI to directly support blocks — i.e., to enable Haskell functions to be marshalled as blocks to C and to enable C blocks to be marshalled as Haskell functions to Haskell land. This extension will be enabled by the language option BlockObjects.

Example: returning a C block

Conversely, a C block object can be used as a function in Haskell. Given the following C prototype

typedef void (^callback_t)(int);
callback_t
get_callback (void);

assume the FFI declaration

foreign import ccall get_callback :: IO (CInt -> IO ())

We might use the imported C function as follows:

do
callback <- get_callback
callback 42

TODO Is there a better example? Something from an official API?

Storage management

TODO How do we recover a Haskell function's storage once the function has been turned into a block object and passed to a C function? (NB: the environment of the function may hold on to large data structures, which will only be freed once the function is freed.)

When we marshal a C block object into a Haskell function, we need to ensure that the Haskell storage manager releases the block object (with Block_release()) once the Haskell land function becomes unreachable in the Haskell heap.

The gory details

The following subpages provide details on implementing this functionality. (The following four subpages are still stubs.)